1. Field of Invention
This invention relates to wavelength-cyclic communication networks and wavelength-cyclic add/drop modules.
2. Description of Related Art
Communication information can be routed between customers using various different kinds of communication networks, including optical fiber communication networks. One type of optical network uses a ring configuration to send information to and receive information from subscribers. In a ring network, nodes which route, terminate, or otherwise process signals are connected by optical communication links, such that the links form a single, closed loop. Optical ring networks may employ wavelength division multiplexing (WDM), in which a plurality of communication channels, in the form of wavelength bands, are combined into a single optical transmission medium, such as an optical fiber. Traffic on such WDM optical rings may be hubbed, in which case all wavelength channels either originate and/or terminate at a central hub node, while one or more wavelength channels are supplied to each subscriber by corresponding add/drop modules that are located at the remote nodes of the ring. Each of the add/drop modules also integrates communication information provided by a corresponding subscriber into the optical communication link so that the subscriber""s communication information is returned to the hub node. For traffic which is not hubbed, a wavelength channel can originate at a first subscriber, be integrated into a communications link by a first add/drop module, be transported to a second add/drop module capable of selecting the corresponding wavelength, and be terminated by a second subscriber. More complex mesh networks can be constructed by combining rings which intersect at one or more nodes.
To provide the communication channels to a subscriber, each of the add/drop modules filter out a particular wavelength channel by either actively selecting a particular wavelength, such as by using a tunable resonant cavity device, or by passively selecting, e.g., by filtering, a specific wavelength channel. Thus, the add/drop modules are capable only of selecting a single particular wavelength channel at any given time, i.e., add/drop modules that actively select a wavelength channel can only select one channel at a time and must be re-tuned to select another channel.
Since the add/drop modules can only select a single wavelength channel at a given time, providing multiple wavelength channels to a particular subscriber either requires that an active add/drop module actively select and provide the wavelength channels for the subscriber, or that multiple passive add/drop modules be provided so that multiple channels can be selected and provided to the subscriber. Active add/drop modules are typically expensive and complex, and require continual monitoring and network management. Single-channel passive add/drop modules are simple and reliable, but adding additional passive add/drop modules to provide expanded service to a subscriber usually requires a disruption in service when the additional equipment is installed. Since each single-channel passive add/drop module weakens the optical signal (i.e. contributes optical loss to the ring), the total number of channels provided in this way is quite limited.
The invention provides a wavelength-cyclic communication network in which each add/drop module in the network is capable of selecting a distinct comb of every Nth wavelength channel for providing to a corresponding subscriber or group of subscribers. The number N can vary depending on the number of add/drop modules included in the network, or other factors such as add/drop modules that may be added to the network in the future. Thus, a first add/drop module in the network can be capable of selecting a first channel, e.g., channel J, as well as channels J+N, J+2N, J+3N, etc. A second add/drop module in the network can select channels (J+1), (J+1)+N, (J+1)+2N, etc. Optionally, the second add/drop module can select the same channels as the first add/drop module, e.g., if the subscribers using the first and second add/drop modules communicate with each other. Thus, each add/drop module has access to M/N channels, where M is the maximum number of channels provided in the network.
Since each add/drop module in the network can be configured to passively select every Nth channel for a corresponding subscriber or group of subscribers, bandwidth upgrades can be performed independently at each add/drop module without effect on any of the other add/drop modules or disruption of service at the add/drop module being upgraded. This is because the add/drop module can be constructed to provide M/N channels without necessarily requiring any additional parts, especially active tuning components. In addition, since each add/drop module provides a comb of wavelength channels that are each separated from adjacent selected channels by Nxe2x88x921 other channels, selection filters used to separate multiple channels in a drop fiber can have lower wavelength resolution. Accordingly, a basic implementation having only one channel per subscriber will use transmitters having a narrow band of wavelengths, thus minimizing initial component cost. For example, a ring network with 16 or fewer subscribers could begin service using only wavelengths for which components are commercially available, and reserve use of wavelengths for which components are not readily commercially available for the future.
The wavelength-cyclic add/drop modules capable of selecting a comb of every Nth wavelength channel can be constructed in various different ways, including cascades of Mach-Zehnder interferometers, back-to-back waveguide grating routers, multiple single channel add/drop modules that are connected to provide the desired wavelength selection features, or Fabry-Perot interferometers.
The invention also provides a Fabry-Perot interferometer that has a relatively simple construction, yet provides passive selection of every Nth wavelength channel, with the accurate tuning (i.e., placement of the wavelength channels) required for use in a wavelength-cyclic network.
The Fabry-Perot interferometer can be constructed in different ways to have the desired add/drop features. In one embodiment, the interferometer includes a pair of reflective surfaces that are separated by at least two portions that have different optical dispersion properties. The different dispersion properties allow desired tuning of the interferometer so that a desired set of every Nth wavelength channels is selected. The reflective surfaces can be formed from metallic films, multilayer dielectric structures or other reflective materials or material combinations. The portions of the interferometer between the two mirror surfaces can include a silica portion and an air portion that each have a desired thickness. By varying the silica and air thickness, the interferometer can be tuned to select a distinct comb of wavelength channels. The region between the mirror surfaces can also include an optional antireflection layer that is formed, for example, on a portion of a silica layer. The antireflection layer can be helpful in suppressing reflections, for example at an air-silica interface, that would otherwise complicate the wavelength response of the device. However, the antireflection layer is not required.
As another example, the Fabry-Perot interferometer can be constructed to have two or more mirrors within the Fabry-Perot cavity, i.e., multiple mirrors between two nearly parallel reflecting structures. The additional mirrors fold the light path within the device, adjusting the phase of the light at each mirror reflection and allowing full tuning of the device, i.e., the device can be constructed to select a desired set of every Nth wavelength channels. Polarization independence in the device can be achieved either by having four total angled mirrors within the interferometer cavity, or by using polarization diversity techniques. Using polarization diversity, an incoming light signal is split into two beams of orthogonal polarization, one beam is polarization rotated, both beams pass through the device in parallel and the beams are recombined at the output of the device. Devices using polarization diversity techniques to achieve polarization independence only require two mirrors or reflective surfaces within the interference cavity.
Tuning of the interferometers allows the interferometers within a single ring network to select different combs of wavelength channels, i.e., none of the add/drop modules within a same ring network will unintentionally select a same wavelength channel. Thus, a first add/drop module can be constructed to select channels J, J+N, J+2N, etc. A second add/drop module can be constructed to select channels (J+1), (J+1)+N, (J+1)+2N, etc. Accordingly, upgrading, i.e., providing additional wavelength channels, to a subscriber from a particular add/drop module can be done without disruption of service for the subscriber or any other subscribers within the ring network. This is because each add/drop module can be constructed to select every Nth wavelength channel without requiring active channel selectors or multiple single-channel selection devices, filtering devices, etc.
These and other aspects of the invention will be apparent and/or obvious from the following description.